Hyperspectral sonar

ABSTRACT

A sonar survey system and method excites reflectors using a broadband message that excites all frequencies within the band providing for selection and evaluation of frequencies of interest after the survey is completed.

PRIORITY APPLICATIONS AND INCORPORATION BY REFERENCE

This application is a continuation of Ser. No. 16/118,327 filed Aug. 30,2018 entitled Hyperspectral Sonar which is a continuation of Ser. No.15/700,482 filed Sep. 11, 2017 entitled Hyperspectral Sonar, now U.S.Pat. No. 10,067,228. This application incorporates by reference, intheir entireties and for all purposes, the disclosures of U.S. Pat. No.3,144,631 concerning Mills Cross sonar, U.S. Pat. No. 8,305,841concerning sonar used for mapping seafloor topography and U.S. Pat. No.3,488,445 concerning matched filtering of orthogonal signals.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to underwater acoustical systems, methodsfor using underwater acoustical systems, and methods for processing andusing the data they produce. In particular, the invention relates tohyperspectral survey systems.

Discussion of the Related Art

Sonar survey systems that operate at one or at a few frequencies areknown in applications including bathymetric surveys and bottomclassification. However, although broadband survey systems can also beused for bathymetric surveys, their use for sea floor and water columnclassification are generally unknown.

SUMMARY OF THE INVENTION

The present invention provides a hyperspectral sonar system. With goalssimilar to bottom characterization and water column characterizationmissions, hyperspectral sonar uses substantially different transmitwaveforms, beamforming, and post-processing methods to acquire dataacross a broad spectrum of frequencies.

Hyperspectral operation can be contrasted with multi-spectraltechniques. In the one, multi-spectral, acoustic data are acquired atparticular frequencies such that results are available only at thosefrequencies in use during the original data acquisition. Frequencies ofinterest to the user must be known and chosen ahead of time prior todata collection. In the other, hyperspectral, data are acquired across abroad frequency range. Contemporaneous or post processed hyperspectraldata therefore provide results at any frequencies of interest up to theNyquist frequency of the system with a frequency resolution of1/acquisition time. Compared to the multi-spectral case, hyperspectraltechniques offer higher frequency resolution when collectingfrequency-dependent results and provide the user flexibility in choosingfrequencies of interest in post-processing; no prior knowledge isrequired.

Like the multi-spectral system, the minimum and maximum operatingfrequencies of a hyperspectral system are determined by the operatingband of the sonar. However, unlike multi-spectral systems that utilizemultiple sonar systems for transmitting multiple waveforms,hyperspectral sonar has no such requirement; a single hyperspectralsystem will suffice. Furthermore, no library of transmit waveforms withdifferent center frequencies is required since one broadband waveformmay substitute for all.

Applicant notes that some known sonar systems may utilize broadbandsignals. However, these systems do not operate like applicant'shyperspectral sonar system. For example, these systems may have one ormore of the following characteristics which distinguish them fromembodiments of applicant's invention:

-   -   operate at low frequencies less than 20 kHz;    -   have a relatively small bandwidth spanning less than a decade of        frequencies;    -   be single beam systems with one broad beam of around 10 degrees        beamwidth in one dimension;    -   be oriented such that beam(s) are horizontal;    -   use pulse compression or matched filtering techniques; and    -   classify targets based on resonance.

Take, for example, broadband sonar systems used in fishing applications.Such systems possess many of the above qualities and are incompatiblewith applicant's hyperspectral sonar system. A fisheries sonar typicallyuses a wide beam to integrate the combined backscatter over a largevolume of water for estimating total biomass in the volume, andinterpretation of the local minima and maxima of the returned spectrumallows for species estimation based on known resonances of fish. Tocontrast, a hyperspectral system focuses a very narrow beam (forexample, 1°×1° beamwidth) on discrete targets, making it impractical forintegrations over large volumes. Hyperspectral systems also excludematched filters and examine echo content at only specific frequencies ofinterest.

In an embodiment, applicant's survey system provides: a vertical surveysystem including a broadband multibeam echo sounder system that avoidsthe use of matched filters for installation on a water going vehicle; anacoustic transceiver for use with one or more transducers in a singleprojector array and plural transducers in a single hydrophone array; theprojector array arranged with respect to the hydrophone array to form aMills Cross; the system capable of forming beams with 1°×1° or betterresolution; a transceiver for synthesizing a transmitter messageincluding a frequency modulated (FM), noise-like, click, or click trainmessage with a frequency between 20 and 1000 kHz and with a bandwidth of100 kHz or more; and, the message for exciting the projector array suchthat a 180 degree or smaller swath of a waterbody bottom is ensonifiedby the message, and a message echo from ensonified scattering centers isreturned to the hydrophone array; wherein data derived from thehydrophone returns are stored and subsequently made available foranalysis of any frequency within the band using narrow band pass filtersor spectral analysis and comparison of the results from any two or morefrequencies.

In an embodiment, applicant's survey system provides: A method ofcomparing band pass filter outputs to assess the strength of echoreturns at particular frequencies, the method comprising the steps ofacquiring multibeam sonar data and indexing the data by ping number,beam number, and sample number; storing the sonar data; selecting a pingand selecting a beam of the ping; processing one beam at a time byidentifying a time series associated with the beam, using no matchedfilters, selecting a band pass filter centered at a discrete frequencyf_(i) where the selected band pass filter frequency need not match thefrequency of the transmitted waveform, processing the time seriesthrough a band pass filter centered at discrete frequency f_(i), andrepeating the processing step for n frequencies where n is two or more;and, comparing the band pass filter outputs at two or more band passfilter frequencies.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described with reference to the accompanyingfigures. These figures, incorporated herein and forming part of thespecification, illustrate embodiments of the invention and, togetherwith the description, further serve to explain its principles enabling aperson skilled in the relevant art to make and use the invention.

FIG. 1 shows a sonar system of the present invention 100.

FIG. 2A shows a message timing diagram 200A.

FIG. 2B-C show multispectral messages in parallel and serial format200B-C.

FIG. 2D shows a multispectral survey system 200D.

FIGS. 3A-C show hyperspectral messages and frequencies of interest300A-C.

FIGS. 4A-B show hyperspectral messages in an 80 to 260 kHz band 400A-B.

FIGS. 4C-D show hyperspectral survey systems 400C-D.

FIGS. 4E-G show hyperspectral messages and sub-bands 400E-G.

FIG. 4H shows a modification to the survey system of FIG. 4D enablingoperation with sub-bands 400H.

FIG. 5 shows hyperspectral mission types 500.

FIG. 6A shows a hyperspectral survey process using band pass filters600A.

FIG. 6B shows a hyperspectral survey process using Fourier analysis600B.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The disclosure provided in the following pages describes examples ofsome embodiments of the invention. The designs, figures, and descriptionare non-limiting examples of the embodiments they disclose. For example,other embodiments of the disclosed device and/or method may or may notinclude the features described herein. Moreover, described features,advantages or benefits may apply to only certain embodiments of theinvention and should not be used to limit the disclosed invention.

As used herein, the term “coupled” includes direct and indirectconnections. Moreover, where first and second devices are coupled,intervening devices including active devices may be locatedtherebetween.

Frequency Dependence of Sonar Returns

FIG. 1 shows a sonar system 102 that ensonifies a location on an oceanfloor 108. The sonar system 102 emits an acoustic signal or message 104from projector transducers and the signal strikes scattering centers orreflectors 108 at the ocean floor 110. Returns from the acousticinteraction 106 are received by hydrophone transducers 107 at the sonar102.

The acoustic interaction between the emitted signal and the reflectorproduces a return that depends on a number of variables including angleof incidence and frequency of the emitted signal. For example, when thesurvey system is directly over the reflector the backscatter strengthmay be a maximum with lesser angles of incidence having less backscatterstrength.

Seabed conditions may be of interest. In the case of varying frequency,at low frequencies where acoustic wavelengths are larger than the scaleof seabed roughness, the seabed surface may appear to be acousticallysmooth. On the other hand, at high frequencies such that acousticwavelengths are smaller than the scale of seabed roughness, scatteringcan dominate the returning signal and the seabed may be consideredacoustically rough. Water column conditions may be of interest. Here,low frequencies with large acoustic wavelengths may be required to matchthe scale of water column features. At this scale, water column featuressuch as fish come to mind. Where water column features such asparticulate, small bubbles, and plankton are of interest, higherfrequencies with smaller acoustic wavelengths may be required to matchthe scale of the feature.

Where conditions of the ocean bottom or water column suggest that theresponse of reflectors to disparate frequencies will be different owingfor example to differing hardness (acoustic reflection coefficient),roughness (as a backscatter coefficient) or scale of an obstruction,multispectral sonar systems may be used to distinguish among types ofbottoms or water column obstructions encountered.

Multispectral Sonar Systems

FIG. 2A shows a message cycle 200A. In particular, the sonar systememits a single ping acoustic message during t1. And, during t3 the sonarsystem receives returns from the emitted message. Between these twotimes there is an optional wait time t2.

FIG. 2B illustrates a multispectral sonar system that acquiresmultifrequency data 200B. In particular, three CW signals at widelyspaced frequencies 50, 150, 250 kHz are included in a transmittedmessage. As shown, the signals are arranged to occur during the sametime span t11-t22. Notably, in some embodiments the signals are ofdiffering durations.

FIG. 2C illustrates a multispectral sonar system that acquiresmultifrequency data 200C. In particular, three CW signals at widelyspaced frequencies 50, 150, 250 kHz are included in a transmittedmessage. As shown, the signals are arranged serially such that theyoccur in time spans t11-t22, t22-t33, and t33-t44. Notably, in someembodiments there are temporal gaps between the signals.

Sonar systems capable of constructing the messages of FIG. 2B-C,ensonifying a target with the message, and processing the returnsinclude multispectral systems that operate at the three frequencies.These sonar systems must segregate the signals at the three frequencies,and bandpass filters or matched filters or their equivalents aretypically used for this purpose.

Multispectral sonar systems such as those described above may bedistinguished from hyperspectral sonar systems. In particular,hyperspectral sonar systems utilize a broadband signal to excite allfrequencies within the band for ensonifying a target and have no need ofsegregating signals at various frequencies during transmission as wasthe case for multispectral sonar systems.

Broadband signals are contrasted with narrow band signals. In anembodiment, a signal is a broadband signal when it is not a narrow bandsignal.

In an embodiment, broadband signals occur in the range of 20 kHz to 1000kHz and may have bandwidths in the range of 20 kHz to 1000 kHz.

In an embodiment, a signal is a broadband signal when it exceeds 20 kHzand its bandwidth is more than 10% of the center frequency.

In an embodiment, a signal is a broadband signal when a statisticallysignificant difference exists between a) an acoustic return from atarget excited by a first frequency in the band and b) an acousticreturn from the target excited by a second frequency in the band.

In an embodiment, a signal is a broadband signal when the backscatterstrength at first frequency in the band differs by more than aprescribed amount from the backscatter strength at a second frequency inthe band. In some embodiments a difference of about 2 dB or more mayindicate that the signal is a broadband signal. An example follows.

Consider a message comprising signals Sx in frequency bands Bx and Bywhere Bx is the lower of the two frequency bands. Where backscattersignal strengths BSx and BSy differ by 2 dB or more, then the signal isa broadband signal.

Any one or more of the above described methods may be used to determinewhether a signal is a broadband signal.

FIG. 2D shows a multispectral survey system 200D. The echo soundersystem includes a transducer section 220, a transmitter section 250, anda receiver section 270. Some embodiments include an interface section290 and/or a management section 292.

Notably, N may vary from 2 to the number of different center frequenciesto be transmitted. In the embodiment shown, N=2, so a message 253incorporating first and second signals S_(cf1), S_(cf2) at first andsecond different center frequencies f1, f2 is used to excite threeprojectors in a projector array. A receiver having three hardwarepipelines and six software pipelines is used to process T hydrophonesignals for recovery of echo information specific to each of Nfrequencies. Note that T is the number of hydrophones and that here,T=3.

The transmitter section 250 is for exciting the projector array 230. Thesection includes a signal generator block 258, a transmit beamformerblock 256, a summation block 254, and a power amplifier block 252.

In the signal generator block 258, N signal generators are shownoperating at different user selectable center frequencies f1, f2. Inrespective beamformers of the beamformer block 256, multiple beams aregenerated from each signal. In a summation block 254, the beams arecombined to produce a summation block output signal 253.

The transducer block 220 includes a projector array 230 and a hydrophonearray 240 arranged as a Mills Cross. As shown, there are threeprojectors 231 in the projector array and three hydrophones 241 in thehydrophone array. In the power amplifier block 252, the summed signal ormessage 253 is an input to power amplifiers driving respectiveprojectors.

Applicant notes that for convenience of illustration, the projector andhydrophone counts are limited to three. As skilled artisans willappreciate, Mills Cross arrays need not have equal numbers of projectorsand hydrophones nor do the quantities of either of these transducersneed to be limited to three. For example, a modern multibeam echosounder might utilize 1 to 96 or more projectors and 64 to 256 or morehydrophones.

The array of T hydrophones 241 is for receiving echoes resulting fromthe acoustic/pressure waves originating from the projector array 230.The resulting hydrophone signals are processed in the receiver section270 which includes a hardware pipeline block 272, a software pipelineblock 274, a receive beamformer block 276, and a processor block 278.

In the hardware pipelines block 272, each of T hardware pipelinesprocesses a respective hydrophone 241 signal through analog componentsincluding an analog-to-digital converter. In the embodiment shown, ahardware pipeline provides sequential signal processing through a firstamplifier, an anti-aliasing filter such as a low pass anti-aliasingfilter, a second amplifier, and an analog-to-digital converter.

In the software pipelines block 274, each of the T hardware pipelineoutputs is processed through N software pipelines with downconversionand matched filtering. In the embodiment shown, a software pipelineprovides sequential signal processing through a mixer (oscillator is notshown for clarity), a bandpass filter, a decimator, and a matchedfilter. Communications may occur via communications links between any ofthe processor block 278, the signal generator block 258, the hardwarepipelines block 272, the software pipelines block 274, the and thebeamformer block 276. See for example FIG. 2D.

In the receive beamformer block 276, each of N beamformers processessignals. As such, three software pipeline outputs at a first centerfrequency are processed by a first beamformer and three softwarepipeline outputs at a second center frequency are processed by a secondbeamformer. Notably, beamformers may be implemented in hardware orsoftware. For example, multiple beamformers may be implemented in one ormore field programmable gate arrays (“FPGA”).

In the processor block 278, each of N processors are for processingrespective beamformer outputs. Here, a first plurality of beamsgenerated by the first beamformer is processed in a first processor anda second plurality of beams generated by the second beamformer isprocessed in a second processor. Processor outputs interconnect with amanagement section 292. Notably, one or more processors may beimplemented in a single device such as a single digital signal processor(“DSP”) or in multiple devices such as multiple digital signalprocessors.

Complementary data may be provided via a sensor interface section 290that is interfaced with a plurality of sensors ES1, ES2, ES3. The sensorinterface module may provide sensor data to management section 292and/or to processors in the processor block 278.

In an embodiment, management section 292 and sensor interface section290 are provided. The management section includes an interface module294 and/or a workstation computer 296. The sensor interface sectionprovides for interfacing signals from one or more sensors ES1, ES2, ES3such as sensors for time (e.g. GPS), motion, attitude, and sound speed.

The management section 292 includes a sonar interface 294 and/or aworkstation computer 296. In various embodiments control signals fromthe management block 292 are used for one or more of making poweramplifier block 252 settings (e.g., for array shading), controllingtransmit 256 and receive 276 beamformers, selecting software pipelineblock 274 operating frequencies, setting set signal generator block 258operating frequencies, and providing processor block 278 operatinginstructions.

As shown in FIG. 2D above, the projectors are driven by a signal atdiscrete frequencies. Returns received by the hydrophones are mixed withrespective oscillator signals and the signals, for example thedifference signals, are passed through respective band pass and matchedfilters.

Hyperspectral Sonar Systems

FIG. 3A shows a hyperspectral broadband signal 300A. As seen, thebandwidth of the signal is from 100 to 700 kHz, a range of 600 kHz.

Referring again to the transducers of FIG. 1 103, 107, these projectorsand hydrophones have a much wider and higher operating band than thatused by conventional sonar systems that may also use broadbandwaveforms. In particular, these projectors and hydrophones may operatein a frequency range of 20 to 1000 kHz. Furthermore, the spacing oftransducer elements in the transmit and receive arrays is selected toallow beams to be formed with 1-degree resolution.

FIG. 3B shows the hyperspectral signal of FIG. 3A used to ensonify atarget 300B. In operation, the hyperspectral sonar ensonifies a targetwith this signal such that the target is excited with all of thefrequencies in the band 100 to 700 kHz during time span t11-t22.

FIG. 3C shows the hyperspectral signal of FIG. 3B and exemplary centerfrequencies of interest 314 (300 kHz, 400 kHz, 600 kHz). Notably, withhyperspectral sonar the center frequencies of interest can be chosenafter the data are acquired because data for all of the frequencies inthe band are returned.

If the available bandwidth of the sonar equipment is less than thatneeded for the hyperspectral signal, the sonar may be adapted byaggregating bands. FIGS. 4A-D below illustrate the case of adequatesonar equipment bandwidth while FIGS. 4E-H illustrate the case ofaggregated bands to accommodate limited bandwidth.

Hyperspectral Sonar, No Bandwidth Limitation

FIG. 4A shows a hyperspectral broadband signal 400A. As seen, thebandwidth of the signal is from 80 to 260 kHz, a range of 180 kHz.

FIG. 4B shows the hyperspectral signal of FIG. 4A used to ensonify atarget 400B. In operation, the hyperspectral sonar signal 404 ensonifiesa target such that the target is excited by all of the frequencies inthe band 80 to 260 kHz during time span t11-t22.

FIG. 4C shows an embodiment including a single beam hyperspectral sonarsystem 400C. The sonar system includes a signal generator 458 forconstructing a message that is delivered to an amplifier 452 andthereafter to a projector 431. Returns from the message are picked up bya hydrophone 441 and processed. The projector and hydrophone maycomprise an integral unit. There may be one or several projectors and/orhydrophones, and one transducer may act as both a projector and ahydrophone.

Processing occurs in a processing pipeline 470. The processing pipelineincludes a first pipeline block 472 which may be hardware or softwareand a second pipeline block 474 which may be hardware or software. In anembodiment, the first block includes hardware and in an embodiment thesecond block includes software. In an embodiment, the first blockconsists of hardware and in an embodiment the second block consists ofsoftware.

The first pipeline block 472 may include one or more of amplificationfollowed by anti-aliasing which is again followed by amplification andanalog to digital conversion.

The second pipeline block 474 may include one or more of mixing followedby bandpass filtering followed by decimation. An oscillator may beconnected with the mixer as shown. The first pipeline block 472 feedsthe second pipeline block 474 which feeds facilities for processing andstorage 410 at interface 412. In various embodiments, the processing andstorage facility receives a beam time series via the pipelines.

The processing and storage facilities 410 may include processing,storage, or processing and storage. Processing of the second pipeline474 output may precede storage. Storage may include the beam time seriesor data derived from the beam time series. In various embodiments, thestorage provides a means to examine the returns at any frequency withinthe band encompassed by the broadband message emitted by the projector431.

As shown, the processing and storage facility 410 may include any of aprocessor 414, a real time output 420, and a storage section 430. In anembodiment, the processing and storage facility interface 412 receivesthe output of the second pipeline block 474 and may exchange signalswith a sensor interface 490. The sensor interface section provides forinterfacing signals from one or more sensors ES1, ES2, ES3 such assensors for time (e.g. GPS), motion, attitude, and sound speed.

In the processor block 414 calculations may be performed includingcalculations measuring round trip travel time for the acoustic signalemitted by the projector 431. Processor outputs may interconnect with amanagement section 435. Notably, one or more processors may beimplemented in a single device such as a single digital signal processor(“DSP”) or in multiple devices such as multiple digital signalprocessors.

The management section 435 may include a sonar interface and aworkstation computer. In various embodiments control signals from themanagement section are used for one or more of making power amplifierblock 452 settings, selecting software pipeline block 474 operatingfrequencies, setting signal generator block 458 broadband messageparticulars, generating signals 458, and providing processing andstorage facility 410 instructions. In various embodiments the processorblock 414 and management section 435 may share processors.

FIG. 4D shows an embodiment including a multibeam hyperspectral sonarsystem 400D. While this sonar system is similar to the multispectralsonar system described above, there are important differences.Differences include changes made in the transmitter section 450, thesecond pipeline block 474, in the beam forming section 476, and in theprocessor section 478.

In the transmitter section, a single broadband signal is generated 456.This signal is subsequently passed through a beam former 454. The beamformer output or a signal derived therefrom is amplified in poweramplifier(s) 452. For example, three projectors are shown and eachprojector is driven by a respective power amplifier.

In the second pipeline block 474, a first difference is the eliminationof matched filters. A second difference is the elimination of themultispectral aspect that provided for each hydrophone twomixing/filtering sections to handle returns from a transmittedmultispectral message including content at a first frequency and contentat a second frequency. In the drawing, a mixer 4741 is followed by anotch filter 4742 and a down converter 4743. An oscillator is for usewith each of the mixers 4744. In an embodiment, each pipeline has anoscillator and the oscillator may provide a common oscillator frequencyto each mixer. In an embodiment, the oscillator signal is provided by acommon oscillator.

In the beam forming section 476, two beam formers were required wherethe multispectral system transmitted a message with two differentfrequency components. However, in the hyperspectral system only a singlebeam former is required to process returns from a broadband message.

In the processing and storage facility 410, a signal interface 412receives the beam former output and may be interconnected with one ormore of a processor 414, a real time unit 420 and a storage unit 430.

Complementary data may be provided via the interface 412. For example, asensor interface section 490 may provide for interconnection with aplurality of sensors ES1, ES2, ES3. Sensors may include sensors fortime, motion, altitude, and sound speed.

In some embodiments, a management section 435 is provided. Themanagement section may include a sonar interface and/or a workstationcomputer. In various embodiments control signals from the managementsection are used for one or more of making power amplifier block 452settings (e.g., for array shading), controlling transmit 454 and receive476 beamformers, selecting software pipeline block 474 operatingfrequencies (where broadband signals are subdivided), setting signalgenerator block 456 signal particulars including a start frequency andbandwidth, and providing processor block 478 operating instructions.

As shown in FIG. 4D above, the projectors are driven by a singlebroadband source. Returns received by the hydrophones are mixed withrespective oscillator signals and the signals are passed throughrespective band pass filters before reaching the beamformer. No matchedfilters are used.

Hyperspectral Sonar, Bandwidth Limited

FIG. 4E shows a hyperspectral broadband signal 400E. As seen, thebandwidth of the signal is from 80 to 260 kHz, a range of 180 kHz. But,as seen this bandwidth is divided into three 60 kHz sub-bands.

FIG. 4F shows a serial hyperspectral signal 400F incorporated in amessage. A low band 4120 of 60 kHz adjoins an intermediate band 4140 of60 kHz which adjoins a high band 4160 of 60 kHz such that the 180 kHzrange is covered. As seen, the sub-bands are transmitted in serialfashion with the low band transmitted in time span tt1-tt2, theintermediate band transmitted in time span tt2-tt3 and the high bandtransmitted in time span tt3-tt4.

FIG. 4G shows a parallel hyperspectral signal 400G incorporated in amessage. A low band 4252 of 60 kHz adjoins an intermediate band 4254 of60 kHz which adjoins a high band 4256 of 60 kHz such that the 180 kHzrange is covered. As seen, the sub-bands are transmitted in parallelfashion in time span tt11-tt22.

FIG. 4H shows a modification 400H to the multibeam hyperspectral sonarsystem of FIG. 4D that accommodates sub-bands. As seen, the analog todigital converter outputs are passed to branches that are filtered viabandpass filters to accommodate each of the sub-bands. Notably, eachbranch includes a mixer, a bandpass filter, and a down converter.Oscillators interconnected with the mixers are omitted for clarity.

In this example, there are three sub-bands. Therefore, each analog todigital converter feeds three branches. And, each branch has a bandpassfilter that selects a sub-band. Bandpass filters are therefore providedfor 80 to 140 kHz, for 140 to 200 kHz, and for 200 to 260 kHz.

After processing in the branches, the output of each analog to digitalconverter is summed. The summed outputs are then passed to thebeamformer.

FIG. 5 shows a mission table 500. Bottom missions may include bottomclassification and/or segmentation, sub-bottom classification and/orsegmentation, and improved bottom detection. Other missions may includewater column classification and/or segmentation, and objectclassification. All of these missions may be carried out with broadbandmessages including either of frequency modulation (FM), noise-like,click, or click train signals.

Bottom classification involves partitioning acoustic returns from theseafloor into discrete groups associated with various sediment or bottomtypes. Variations in backscatter strength from multiple narrowbandfrequencies are used to construct a feature set representing sea floorresponse versus frequency or versus frequency and incident angle. (Itshould be noted that there is an upper frequency above which backscatterstrength ceases to vary with frequency.) These features can then be usedindependently to segment bottom types or they can be correlated toexpected responses from known sediment types in attempts to characterizethe sea floor.

During a bottom classification and/or segmentation mission, a broadbandmessage in the range of 20 to 800 kHz may be used. This message may havea bandwidth of 100 kHz or greater.

Sub-bottom classification is similar to bottom classification in its useof variations in acoustic returns from multiple narrowband frequenciesto characterize sea floor types, but its use of lower frequency soundsallows acoustic penetration into the sea floor. Sub-surfaceclassification can characterize sediment types, rocks, or fluids beneaththe sea floor.

During a sub-bottom classification and/or segmentation mission, themessage frequency will tend toward the lower end of the frequency range.A broadband message in the range of 20 to 100 kHz may be used. Thismessage may have a bandwidth of 40 kHz or less.

During an improved bottom detection mission, a broadband message in therange of 20 to 800 kHz may be used. This message may have a bandwidth of100 kHz or greater. Dominant or predictable clutter frequencies can beignored in favor of those with high backscatter strength from a knownbottom type.

Water column classification is a method for detecting and identifyingaspects of the marine environment between the ocean surface and seafloor. Naturally occurring content in the water column can includeanimals, plants, dust, and bubbles. Frequency-dependent features of theacoustic backscatter from these objects can be used to identify or inferproperties of the features causing the backscatter.

During a water column classification and/or segmentation mission, abroadband message in the range of 20 to 800 kHz may be used. The messagemay have a bandwidth of 100 kHz or greater.

During a mission to classify objects, for example manmade objects fromnatural clutter objects, sonar resolution may be insufficient forclassification. In such cases, variations in target response fromdifferent narrowband frequencies can be provided to automatedclustering, segmentation, or classification applications.

During an object classification mission, a broadband message in therange of 20 to 800 kHz may be used. The message may have a bandwidth of100 kHz or greater. The message may have a bandwidth of 100 kHz orgreater.

FIG. 6A shows a hyperspectral sonar method using band pass filters 600A.In a first step, data acquired by the sonar system including one or moreacoustic returns are indexed by ping number, by beam number, and bysample number 602.

In a second step 603 the data are stored. The data may be usedcontemporaneously or the data may be used at a later time, for exampleto evaluate the strength of the returns at various frequencies.

In a third step, a particular ping is selected 604. In a fourth step, aparticular beam is selected 606. In a fifth step, the time seriesassociated with the particular beam is identified 608.

In a sixth step, a band pass filter range is selected 610. In a seventhstep, the time series is passed through the band pass filter 612. In aneight step 614, the band pass filter output is the frequency contentwithin the pass band.

Notably, the width of the band pass filter may be a fraction of thecenter frequency. For example, the width of the band pass filter may be10% of the center frequency. For example, the width of the band passfilter may be 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8% or 9% of the centerfrequency. In an embodiment, the width of the band pass filter is 2 kHzor between 2 and 10 kHz.

In a ninth step, a query asks if enough band pass filter ranges havebeen selected 616. If so, the process subsequently compares band passfilter outputs in step 620. If not, the process selects another bandpass filter range 618 and subsequently moves to step 612.

FIG. 6B shows a hyperspectral sonar method using Fourier Transforms600B. In a first step, data acquired by the sonar system including oneor more of acoustic returns or travel times is indexed by ping number,by beam number, and by sample number 652.

In a second step 653 the data are stored. The data may be usedcontemporaneously or the data may be used at a later time, for exampleto evaluate the strength of the returns at various frequencies.

In a third step a particular ping is selected 654. In a fourth step, aparticular beam is selected 656. In a fifth step, the time seriesassociated with the particular beam is identified 658.

In a sixth step, a Fourier transform or fast Fourier transform is usedto process the time series 660. In a seventh step the Fourier transformoutput is a spectrum of the time series indexed by frequency 662.

In an eighth step, the spectrum provides the relative strength of theecho at each of multiple quantized frequencies 664. In a ninth step, thespectrum at the frequency bins of interest are compared 666.

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not limitation. It will be apparent to those skilledin the art that various changes in the form and details can be madewithout departing from the spirit and scope of the invention. As such,the breadth and scope of the present invention should not be limited bythe above-described exemplary embodiments, but should be defined only inaccordance with the following claims and equivalents thereof.

What is claimed is:
 1. A hydrographic sonar survey system comprising: abroadband multibeam echo sounder that avoids the use of matched filters;a plurality of transducers in a projector array and a plurality oftransducers in a hydrophone array; the projector array configured toform an elongated transmit beam and the hydrophone array configured toform elongated receive beams; the transmit beam intersects the receivebeams; the system configured to include an operating mode in which theplurality of transducers in the projector array emit a broadbandwaveform; the hydrophone array configured to receive echoes fromscattering centers ensonified by the broadband waveform; and, wherein a)echoes received by the hydrophones result in data that are stored, b)subsets of the data are filtered via band pass filters or Fouriertransforms to produce filtered subsets or transformed subsets, c) atleast two of the filtered subsets or transformed subsets are compared,and d) information derived from the comparison are made available forsubsequent use.
 2. A hydrographic sonar survey system comprising: abroadband multibeam echo sounder that avoids the use of pulsecompression; a plurality of transducers in a projector array and aplurality of transducers in a hydrophone array; the projector arrayconfigured to form an elongated transmit beam and the hydrophone arrayconfigured to form elongated receive beams; the transmit beam intersectsthe receive beams; the system configured to include an operating mode inwhich the plurality of transducers in the projector array emit abroadband waveform; the hydrophone array configured to receive echoesfrom scattering centers ensonified by the broadband waveform; and,wherein a) echoes received by the hydrophones result in data that arestored, b) subsets of the data are filtered via band pass filters orFourier transforms to produce filtered subsets or transformed subsets,c) at least two of the filtered subsets or transformed subsets arecompared, and d) information derived from the comparison are madeavailable for subsequent use.
 3. The system of claim 2 wherein thebroadband waveform type includes frequency modulated (FM), noise-like,click, or click train type waveforms.
 4. The system of claim 2 whereinthe formed beams have a resolution of 1°×1° or better.
 5. The system ofclaim 2 wherein the broadband waveform has a frequency band between 20kHz and 1000 kHz and a bandwidth of about 100 kHz or more.
 6. The systemof claim 2 wherein the broadband waveform has a bandwidth that is morethan 10% of its center frequency.
 7. The system of claim 2 wherein thebroadband waveform produces a statistically significant differencebetween an acoustic return from a target excited by a first frequency inthe band and an acoustic return from the target excited by a secondfrequency in the band.
 8. The system of claim 2 wherein the comparisonincludes mathematical analysis of a first signal with a firstbackscatter strength and mathematical analysis of a second signal with asecond backscatter strength where the difference between the backscatterstrengths is 2 dB or more.
 9. The system of claim 2 wherein thecomparison is influenced by band pass filter outputs to assess thestrength of echo returns at particular frequencies.
 10. A method ofcomparing band pass filter outputs to assess the strength of echoreturns at particular frequencies, the method comprising the steps of:using a plurality of transducers in a projector array to form anelongated transmit beam; transmitting a sonar waveform using theprojector array; using a plurality of transducers in a hydrophone arrayto form elongated receive beams; receiving multibeam sonar data fromlocations where the transmit beam intersects receive beams ; indexingthe data by ping number, beam number, and sample number; storing, now orlater, the received sonar data; selecting a ping and selecting a beam ofthe ping; processing the selected beam by identifying a time seriesassociated with the beam, using no pulse compression, selecting one of aplurality of band pass filters centered at a discrete frequency f1 wherethe selected band pass filter frequency need not match the frequency ofthe transmitted waveform; processing the time series through a band passfilter centered at discrete frequency f1, and repeating the processingstep for n frequencies where n is two or more; and, comparingmathematically the band pass filter outputs at two or more band passfilter frequencies.
 11. The method of claim 10 wherein the mathematicalcomparisons are used for bottom segmentation or characterizationpurposes.
 12. The method of claim 10 wherein the multibeam sonar dataare acquired in response to a broadband waveform.
 13. The method ofclaim 12 wherein the broadband waveform is in a frequency band with abandwidth that is more than 10% of its center frequency.
 14. The methodof claim 12 wherein the broadband waveform produces a statisticallysignificant difference between an acoustic return from a target excitedby a first frequency in the band and an acoustic return from the targetexcited by a second frequency in the band.
 15. The method of claim 10wherein the mathematical comparison includes mathematical analysis of afirst signal with a first backscatter strength and mathematical analysisof a second signal with a second backscatter strength where thedifference between the backscatter strengths is 2 dB or more.
 16. Amethod of assessing the strength of echo returns at particularfrequencies, the method comprising the steps of: using a plurality oftransducers in a projector array to form an elongated transmit beam;transmitting a sonar waveform using the projector array; using aplurality of transducers in a hydrophone array to form elongated receivebeams; receiving multibeam sonar data from locations where the transmitbeam intersects receive beams; indexing the data by ping number, beamnumber, and sample number; storing, now or later, the received sonardata; selecting a ping and selecting a beam of the ping; and, processingthe selected beam by identifying a time series associated with the beam,using no pulse compression, processing the time series in a Fouriertransform such that the Fourier transform output is a spectrum of thetime series indexed by frequency, and via the spectrum, assessing echostrength at multiple quantized frequencies.
 17. The method of claim 16wherein mathematical comparisons of the assessed strengths are used forbottom segmentation or characterization purposes.
 18. A method of bottomclassification comprising the steps of: from a hydrographic sonar surveysystem that avoids the use of pulse compression, transmitting waveformsand receiving echoes where beams formed by a projector array and ahydrophone array intersect; receiving multibeam sonar data includingseafloor acoustic returns in response to a waveform transmitted at acenter frequency of 20 kHz to 1000 kHz with a bandwidth of 100 kHz ormore; and, during post processing, partitioning seafloor acousticreturns into discrete groups associated with similar bottom types;wherein, adjusted for incidence angle, data indicating similarbackscatter strengths at particular frequencies are indicative ofsimilar bottom types; and, wherein the results or information derivedfrom the results are made available for subsequent use.
 19. Ahydrographic sonar system which does not use matched filters comprising:a projector array and a hydrophone array; the projector array forming anelongated transmit beam; the projector array transmitting a singlebroadband waveform for exciting all frequencies within the band; thehydrophone array for forming elongated receive beams intersecting thetransmit beam, the intersections providing sonar data; the sonar data orinformation derived from the sonar data processed by one or more bandpass filters; and, bandpass filter outputs computed mathematically attwo or more bandpass filter frequencies to aid in bottom segmentation orcharacterization.
 20. A hydrographic sonar system which does not usepulse compression comprising: a projector array and a hydrophone array;the projector array forming an elongated transmit beam; the projectorarray transmitting a single broadband waveform for exciting allfrequencies within the band; the hydrophone array for forming elongatedreceive beams intersecting the transmit beam, the intersectionsproviding sonar data; the sonar data or information derived from thesonar data processed by one or more band pass filters; and, bandpassfilter outputs computed mathematically at two or more bandpass filterfrequencies to aid in bottom segmentation or characterization.